By Heather Saul

Trucks carrying volunteers in Iraq have been heading towards the front lines to defend the capital, after Islamist militants overran two cities and warned a battle will rage as it marched onto Baghdad.

Vehicles are seen burned on a street of the northern city of Mosul, Iraq, Thursday, June 12, 2014 (AP)

Trucks carrying volunteers in Iraq have been heading towards the front lines to defend the capital, after Islamist militants overran two cities and warned a battle will rage as it marched onto Baghdad.

Hundreds of young men crowded in front of the main army recruiting centre in Baghdad on Thursday after authorities urged Iraqis to help battle the insurgents.

Fighters of the Islamic State of Iraq and the Levant (Isis) have seized control of Mosul and Tikrit in less than 48-hours, but government forces have managed to stall their advances near Samarra, a city just 110km (68 miles) north of Baghdad.

In Mosul, Iraq’s second biggest city, Sunni militants staged a parade of American Humvees seized from the collapsing Iraqi army, just a day after 500,000 people were forced to flee the area.

Two helicopters, also seized by the militants, flew overhead, witnesses said, apparently the first time the militant group has obtained aircraft in years of waging insurgency on both sides of the Iraqi-Syrian frontier.

State television showed what it claimed was aerial footage of Iraqi aircraft firing missiles at insurgent targets in Mosul. The targets could be seen exploding in black clouds.

Further south, the fighters extended their advance to towns only about an hour's drive from Baghdad, where Shia militia are mobilising in order to protect their country and fight back.

The army of the Shia Prime Minister Nouri al-Maliki's government in Baghdad has essentially evaporated in the face of the onslaught, and abandoned their uniforms and weapons to flee on foot.

Mohammed al Adnani, the spokesperson for Isis, warned "the battle is not yet raging but it will," in a statement posted online. “It will rage in Baghdad and Karbala. So be ready for it. Put on your belts and get ready.”

"The whole of Kirkuk has fallen into the hands of Peshmerga," Jabbar Yawar told Reuters. "No Iraqi army remains in Kirkuk now."

Militants also attacked an Iraqi security checkpoint on Thursday in the town of Tarmiyah, 50 kilometres (31 miles) north of Baghdad, killing five troops and wounding nine, officials told the Associated Press.

Lawmakers tried to hold a session to vote on declaring a state of emergency on Thursday, following a request by the PM, but too few MPs showed up and they were unable to reach quorum to vote.

Britain has deployed a humanitarian team in Iraq to assess the needs of civilians fleeing the violent takeover. The US has said it is “deeply concerned” about the continued aggression and is considering providing further assistance to Iraq in fighting the militants, but did not give further details on what this would entail.

Iran's foreign minister, Mohammad Javad Zarif, offered his country's support to Iraq in its "fight against terrorism" during a phone call with his Iraqi counterpart, Iranian state TV reported.

Iranian President Hassan Rouhani on Wednesday blasted the Islamic State as "barbaric" and said that his country's highest security body will hold an immediate meeting to review the developments in neighbouring Iraq.

It came as the UN Security Council said it deplored the attacks of Tikrit and Mosul "in the strongest terms" and demanded the immediate return of all hostages abducted from the Turkish consulate.

The UN Secretary-General Ban Ki-moon echoed the council's condemnation, saying that "terrorism must not be allowed to succeed in undoing the path towards democracy in Iraq as determined by the will of the Iraqi people."